This application is based on Applications No. 173149 filed in Japan on Jun. 7, 2001, and No. 153102 filed in Japan on May 27, 2002, the content of which is incorporated hereunto by reference.
This invention relates to a low speed electron beam phosphor primarily for use in field emission displays (referred to as FEDs in this patent application).
A FED is a flat panel display which excites phosphor material with low speed electron beams, and is configured with an anode and corresponding cathode. A phosphor film established at the anode-side is excited by electrons emitted from the cathode to cause light emission. The electron beam for excitation at the anode is accelerated by voltages on the order of 0.1 KV to 10 KV. This is a low accelerating voltage in comparison to the several tens of KV typical for cathode ray tubes (CRTs). Therefore, special purpose phosphor material, which is excited by low speed electron beams, is used in FED applications.
Since the accelerating voltage of the electron beam for phosphor excitation in a FED is low compared to a television CRT, electron beam energy for phosphor excitation is low. Low excitation energy electrons cannot cause a phosphor to emit high luminance light. Therefore, compared to a CRT, a FED produces bright light emission by increasing the current density of the phosphor exciting electron beam. If a CRT phosphor is used at high current densities, its lifetime is significantly reduced. Consequently, although various colors of phosphors for use with televisions have been tried, almost none have been usable for FED applications.
(Y, Ce)2O3.SiO2 phosphor has been developed as a phosphor for FED applications allowing high current densities. This phosphor emits blue light. The raw material for a phosphor of this composition is formed by incorporating SiO2 particulates in a mixture of yttrium oxide (Y2O3) and cerium dioxide (CeO2). Phosphor is produced by firing the raw material in a crucible. Phosphor raw material is mixed and fired to result in (Y, Ce)2O3 and SiO2 with a mole ratio of 1, namely with a stoichiometric mixture.
(Y, Ce)2O3.SiO2 phosphor fired in this fashion cannot be formed with a uniform distribution of constituents at the surface and internally. The fired phosphor has excessive SiO2 near the surface of phosphor particles. This is because yttrium oxide (Y2O3) and cerium dioxide (CeO2) form the core of a phosphor particle and SiO2 gradually permeates inward from the surface with firing. Excess surface SiO2 is the cause of electron beam induced luminance degradation for (Y, Ce)2O3.SiO2 phosphor fired in this fashion. A phosphor with high luminance degradation characteristics does not only mean lifetime is shortened when the phosphor is used alone. When used together with other phosphors, it can cause changes in emission colors, For example, (Y, Ce)2O3.SiO2 blue phosphor is used together with (Y, Tb)2SiO5 green phosphor and (Y, Eu)2O3 red phosphor as a white phosphor material. However, luminance and lifetime characteristics of (Y, Ce)2O3.SiO2 blue phosphor are not the same as those of red and green phosphors such as these. As a result, a monochromatic phosphor formed by mixing these kinds of phosphors has the drawback that emission color changes over use.
The present invention was developed to resolve these drawbacks. It is thus a primary object of the present invention to provide a (Y, Ce)2O3.SiO2 phosphor for use with low speed electron beams which has superior lifetime and luminance characteristics.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.
The phosphor for use with low speed electron beams of this invention is represented by the following general composition formula.
(Y, Ce)2O3.nSiO2 
Here, the value of n is in the range 0.4xe2x89xa6n less than 1.0. The value of n in this formula determines the lifetime characteristics of the (Y, Ce)2O3.SiO2 phosphor. A phosphor with a small value of n has improved lifetime characteristics, and a phosphor with large n has worse lifetime characteristics. To improve lifetime characteristics, the value of n should be made small. However, the value of n also affects emission luminance, and values below 0.4 decreases luminance. Consequently, the above mentioned range of n is established considering luminance and lifetime characteristics, and more preferably the range of n is 0.5xe2x89xa6nxe2x89xa60.9.
Further, the phosphor for use with low speed electron beams of this invention can also be represented by the following general composition formula, and the range of values for a and n are given by the expressions below.
(Y1xe2x88x92a, Cea)2O3.nSiO2 
0.001xe2x89xa6axe2x89xa60.05 
0.4xe2x89xa6n less than 1.0 
The value of a in the formula affects the phosphor""s emission luminance and color. If a is either too large or too small, the phosphor""s emission luminance drops off. This is because luminance improvement due to Ce inclusion is ineffective below 0.001, and because optical quenching due to high concentrations occurs above 0.05. The value of a in the composition formula is set considering phosphor luminance and color, preferably in the above mentioned range. A still more preferable range is 0.005xe2x89xa6axe2x89xa60.04.
The (Y, Ce)2O3.SiO2 phosphor for use with low speed electron beams described above is characterized by significant Improvement in lifetime characteristics compared to related art phosphors. This is because excessive SiO2 at the phosphor surface, which causes luminance degradation, is reduced by making the SiO2 to (Y, Ce)2O3 ratio less than 1. The exceptional lifetime characteristics of the (Y, Ce)2O3.SiO2 phosphor described above are shown in Table 1. For example, luminance after 1000 hrs was 45% to 70% of initial luminance for embodiment phosphors. This is radical improvement compared to 30% to 35% for prior art phosphors. In addition, superior emission luminance over prior art phosphors is also shown in Table 1. When luminance of the phosphor of comparison example 1 is taken to be 100%, luminance of embodiments 1 through 35 are considerably improved at 100% to 145%.